The present invention relates to a magnetostrictive torque sensor that detects torque acting on a shaft by sensing magnetostrictive changes.
For example, Japanese Unexamined Patent Publication No. 5-118938 and Japanese Unexamined Patent Publication No. 59-77326 disclose such magnetostrictive torque sensors. A magnetostrictive torque sensor includes a shaft, a magnetostrictive member fitted about the shaft and detecting coil. Torque applied to the shaft deforms the magnetostrictive member, which changes the magnetic permeability of the magnetostrictive member. This varies the flux, or inductance, of the magnetostrictive member. Changes in the inductance are detected based on electromotive force induced in the detecting coil. The torque applied to the shaft is detected accordingly.
As illustrated in FIG. 4, a prior art magnetostrictive torque sensor includes a housing 30, a shaft 32 rotatably supported by the housing 30, a sleeve 33 fitted about the shaft 32 and a magnetostrictive member 34 fitted about the sleeve 33. A cylindrical stator 35 is located about the magnetostrictive member 34. The stator 35 is supported on the shaft 32 by bearings 38. The bearings 38 allow the stator 35 rotate relative to the shaft 32. The stator 35 includes a cylindrical yoke 36 and a pair of retainers 37 for supporting the yoke 36. The yoke 36 has a pair of resin bobbins B. An exciting coil 39 and a detecting coil 40 are wound about each bobbin B. The exciting coils 39 and the detecting coils 40 surround and face the surface of the magnetostrictive member 34. When an alternating current is supplied to the exciting coils 39, the exciting coils 39 generate flux. The flux forms a magnetic circuit through the yoke 36 and the magnetostrictive member 34.
The surface of the magnetostrictive member 34 is divided into two detection regions X and Y. Grooves 34a are formed in each region X, Y. The grooves 34a in the region X are inclined by forty-five degrees relative to the axis of the shaft 32, and the grooves 34a in the region Y are inclined by minus forty-five degrees relative to the axis. When torque is applied to the shaft 32, a compressive force acts on one of the detection regions X and Y and a tensile force acts on the other region depending on the rotational direction of the shaft 32. A tensile force increases the magnetic permeability of the detection regions X, Y and a compression force decreases the magnetic permeability of the regions X, Y. Variations in the magnetic permeability of the magnetostrictive member 34 change the voltages induced in the detecting coils 40. That is, the detecting coils 40 send varying voltage values to a processor (not shown). The processor computes the torque applied to the shaft 32 based on the voltage values from the detecting coils 40.
The yoke 36 is located close to the magnetostrictive member 34 to improve the sensitivity of the torque sensor. To facilitate the installation of the bobbins B (the coils 39, 40) the yoke 36 includes two semi-cylindrical pieces 41, 42, as illustrated in FIG. 5. When installing the coils 39, 40, the bobbins B, about which the coils 39, 40 are wound, are attached to the inner wall of the yoke 36. Thereafter, the pieces 41, 42 are secured to each other along two joints 43 with non-conductive adhesive. Since the yoke 36 is divided into two pieces along a plane that includes the axis and since the non-conductive adhesive is located between the pieces 41, 42, eddy currents in the circumferential direction of the yoke 36 are blocked by the joints 43. The eddy currents, which would otherwise adversely affect the sensitivity of the torque sensor, are reduced. This improves the sensitivity of the torque sensor. However, since the magnetic reluctance of the joints 43 is much greater than that of the other parts, the magnetic symmetry about the axis of the yoke 36 is impaired. The lowered magnetic symmetry of the yoke 36 causes the detection voltage of the torque sensor to change in accordance with the rotational position of the shaft 32. Therefore, even if there is no torque acting on the shaft 32, the torque sensor may erroneously detect that a torque is acting on the shaft 32.
Torque deforms the magnetostrictive member 34, and deformation of the member 34 changes the inductance of the coils. The torque sensor detects torque based on the changes of the inductance. However, the inductance also changes when the distance between the magnetostrictive member 34 and the coils 39, 40 changes. When the distance between the magnetostrictive member 34 and the coils 39, 40 changes, the torque sensor falsely detects torque even if there is no torque acting on the shaft 32. Therefore, it is imperative that the distance between the member 34 and the coils 39, 40 be constant to guarantee the precision of the torque sensor.
As described above, the stator 35 includes the yoke 36 and the two retainers 37, shown in FIG. 4, and the yoke 36 includes two semi-cylindrical pieces 41, 42, shown in FIG. 5. Due to dimensional errors created during assembly, the cross-section of the stator 35 is not completely round. That is, when assembling the pieces 41 and 42 to form the yoke 36, their relative positions may vary. Also, the positions of the retainers 37 may vary relative to the yoke 36. The variation of the relative positions of the parts prevents the inner surface of the retainer 35 from forming a perfect circle about the axis of the shaft 32. Further, the edges of the pieces 41, 42 are exposed on the inner surface of the yoke 36, which disturbs the distribution of the flux.
The resin bobbins B are press fitted in the metal yoke 36. The bobbins B have a greater coefficient of thermal expansion than that of the yoke 36. Therefore, when the temperature of the sensor is relatively high, a stress F is generated between each bobbin B and the yoke 36. The stress F displaces the positions of the exciting coils 39 and the detecting coils 40. Therefore, even if there is no torque acting on the shaft 32, the electromotive force induced in the detecting coils 40 is changed. In other words, the magnetic characteristics of the coils 39, 40 are affected by temperature changes. This degrades the temperature characteristics of the torque sensor thereby lowering the detection accuracy of the torque sensor.
Accordingly, it is an objective of the present invention to provide a torque sensor that accurately detects torque despite temperature changes.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a torque sensor for sensing torque applied to a shaft is provided. The sensor includes a magnetostrictive member, an exciting coil, a detecting coil, a bobbin and stator. The magnetostrictive member is fixed to the shaft is strained by the torque applied to the shaft. The exciting coil generates flux running through the magnetostrictive member. The flux varies in accordance with the strain of the magnetostrictive member. The detecting coil detects the flux variation. The exciting coil and the detecting coil are wound about the bobbin. The stator supports the bobbin such that the coils surround the magnetostrictive member. The stator is rotatably supported relative to the shaft and is formed with a plurality of stator pieces. The stator has an annular groove formed in its inner surface to accommodate w the bobbin. The annular groove has a first end wall, a second end wall and a circumferential wall. The bobbin further has a first end surface, a second end surface and a circumferential surface. The first end surface of the bobbin is fixed to the first end wall of the annular groove. A first annular clearance is formed between the circumferential surface of the bobbin and the circumferential wall of the annular groove. A second annular clearance is formed between the second end surface of the bobbin and the second end wall of the annular groove.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.